SAN FRANCISCO — While pressure to grow will drive more large drug makers to pursue mergers and acquisitions, diminished resources and competition from biotech and specialty pharmaceutical companies will challenge their ability to do so, according to a new report.

The report, by Ernst & Young, looked at the 16 largest drug companies based in the United States, Europe and Japan, as measured by revenue, finding that a slowdown in emerging markets inhibiting their ability to pursue sales there would widen what it called the "growth gap." Comparing IMS Health's forecast for the global drug maker and industry analysts' estimates over the next five years, the Ernst & Young report found that the growth gap would reach about $100 billion by 2015, meaning that it would need an additional $100 billion in revenue that year to keep up with overall market growth.

Slowing growth in developed markets has put sources of organic growth under pressure, the report found, which will likely prompt acceleration in M&A activity this year, but such deals will become harder due to less available operating cash because of slower sales growth — the result of pressure on drug pricing — and increased borrowing to fund higher dividends, stock repurchases and other expenses. E&Y found that the financial capacity for large drug companies to conduct such deals declined by 23% between 2006 and 2012. Meanwhile, the capacity of biotech and specialty pharmaceutical companies — including generic drug makers — has increased by 61% and 20%, respectively. All the while, big drug makers' share of the combined acquisition capacity among the three industry segments has fallen from 85% to 75% over the past six years.

The report predicts more divestitures of various assets by drug companies and more deals offshore and in emerging markets.